1. Field of the Invention
The present invention relates to an optical transmitter, an optical receiver, an optical transmission system, and an optical transmission method. More particularly, this invention is concerned with an optical transmitter, an optical receiver, an optical transmission system, and an optical transmission method for improving the resistivity of an optical transmission system to wavelength dispersion.
2. Description of the Related Art
In high-speed optical fiber communication systems, waveform distortion derived from waveform dispersion occurring along an optical fiber that is a transmission line is a factor of restricting a transmission rate or a distance of transmission. A transmission method making a system resistive to wavelength dispersion and a wavelength dispersion compensation technology are therefore indispensable.
Conventionally proposed transmission methods making a system resistive to wavelength dispersion include an optical duobinary method. For the details of the optical duobinary method, refer to xe2x80x9cCharacteristics of Optical Duobinary Signals in Terabits Capacity, High-spectral Efficiency WDM Systemsxe2x80x9d (Journal of Lightwave Technology, Vol. 16, No. 5, pp. 788-797, May 1998).
According to the optical duobinary method, a binary signal is converted into a ternary signal or any other multilevel signal in order to compress the spectrum of an electric signal. The spectrum of the electric signal is compressed to agree with approximately a half of the spectrum of a non-return-to-zero (NRZ) signal to be transmitted at the same bit rate as a bit rate at which a light signal proportional to the electric signal is transmitted. Consequently, compared with the NRZ signal to be transmitted at the same bit rate, the light signal whose spectrum has been narrowed according to the optical duobinary method is permitted to disperse by approximately twice a larger magnitude while traveling over an optical transmission line.
However, a precoder circuit employed according to the optical duobinary method delays an output of an exclusive OR circuit by a time during which a data signal represents one bit, and feeds it back to one input terminal thereof. As a transmission rate increases, higher precision is required to adjust a delay time. This poses a problem in that it becomes difficult to realize the precoder circuit.
According to the optical duobinary method, an electric signal is converted into a multilevel signal. This poses a problem in that the configuration of a currently commercially available transmitter/receiver for transmitting or receiving the NRZ signal must be modified or optimized.
An object of the present invention is to solve the foregoing problems and to provide an optical transmitter, an optical receiver, an optical transmission system, and an optical transmission method making it possible to obviate a precoder circuit. Moreover, with the optical transmitter, optical receiver, optical transmission system, and optical transmission method in accordance with the present invention, a coding method using a non-return-to-zero (NRZ) signal or a return-to-zero (RZ) signal as a carrier and making an optical transmission system highly resistive to dispersion can be realized.
An optical transmitter according to the present invention comprises an optical power divider, a light modulator, a phase shifter, and an optical coupler. The optical power divider divides input light into first and second continuous wave (CW) lights. The light modulator modulates the first CW light according to a data signal so as to output modulated light. The phase shifter shifts the phase of the second CW light so as to output phase-shifted light. The optical coupler couples the modulated light and phase-shifted light.
An optical receiver according to the present invention comprises a first photo-detector, a clock extracting circuit, and a maximum level detection circuit. The first photo-detector converts a first light signal to be input into an electric signal. The clock extracting circuit extracts a predetermined frequency component of the electric signal and outputs it as a clock signal. The maximum level detection circuit detects the maximum level of the clock signal.
An optical transmission system according to the present invention comprises an optical transmitter, an optical transmission line, and an optical receiver. The optical transmitter outputs a light signal. The light signal is transmitted over the optical transmission line. The optical receiver receives the light signal output over the transmission line. The optical transmitter comprises the foregoing optical transmitter, and the optical receiver comprises the foregoing optical receiver. The optical transmission line includes a control signal transmission line over which the maximum level is transmitted to the optical transmitter.
An optical transmission method according to the present invention comprises four steps. At the first step, light emanating from a light source is divided into at least two division lights. At the second step, the phase of one of the division lights is shifted in order to produce phase-shifted light. At the third step, the other division light is modulated in order to produce modulated light. At the fourth step, the phase-shifted light and modulated light are coupled.